In recent years, a high-resolution display device has been required. For example, full high-definition (the number of pixels is 1920×1080) has been in the mainstream of home-use television devices (also referred to as televisions or television receivers), while high-resolution display devices such as a 4K (the number of pixels is 3840×2160) display and an 8K (the number of pixels is 7680×4320) display have been developed.
A liquid crystal display device is known as a kind of display devices. A transmissive liquid crystal display device adjusts the amount of light from a backlight to be transmitted and shows contrast to display an image by utilizing optical modulation action of a liquid crystal.
As a kind of field-effect transistors, a thin film transistor whose channel formation region is formed using a semiconductor film that is formed over a substrate having an insulating surface is known. Patent Document 1 discloses a technique in which amorphous silicon is used for a semiconductor film that is used in a channel formation region of a thin film transistor. For example, in a liquid crystal display device, a thin film transistor is used as a switching transistor in each pixel.